Blistercoil Weird will get +1/+1 and untap before that instant or sorcery spell resolves.

Blistercoil Weird will get +1/+1 even if the instant or sorcery spell is countered or if Blistercoil Weird is untapped when its ability resolves.

If you copy an instant or sorcery spell on the stack (using Nivix Guildmage’s second ability, for example), the copy isn’t cast. Blistercoil Weird’s ability won’t trigger.